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Re^3: File Reading and Closing confusion

by shmem (Canon)
on Aug 09, 2010 at 00:23 UTC ( #853691=note: print w/ replies, xml ) Need Help??


in reply to Re^2: File Reading and Closing confusion
in thread File Reading and Closing confusion

With $line=<FH>, we need to check for defined-ness.

No. While there are lines read, they are defined since they have $/ attached. If $/ is undef, no point for while, since that's slurp mode. An empty string on the last line with no $/ is - no line.


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Re^4: File Reading and Closing confusion
by repellent (Priest) on Aug 09, 2010 at 03:49 UTC
    I was referring to being wary when $line=<FH> is used on its own. To illustrate:
    $ perl -e 'print "with_ending\nno_ending"' > a.txt $ perl -MData::Dumper open(FH, "<", "a.txt") or die $!; my $line1 = <FH>; my $line2 = <FH>; my $line3 = <FH>; close FH; $Data::Dumper::Useqq = 1; print Dumper [$line1, $line2, $line3]; __END__ $VAR1 = [ "with_ending\n", "no_end", undef ];

    We see that there is a possibility that a line read ($line3 in this case) may return undef. That's what I meant: we need to be wary that a line read will not always return defined.
      I was referring to being wary when $line=<FH> is used on its own.

      Yes, but while takes care of that - and we/you were talking about a while loop, weren't you?

      1.    while (<>) {
      ...
      
      which is Perl short-hand for the more explicitly written version:
      1.    LINE: while (defined($line = <ARGV>)) {
      ...
      

      But of course,

      We see that there is a possibility that a line read ($line3 in this case) may return undef.

      - if you read past EOF, for instance. Then readline returns undef:

      open(FH, "<", "a.txt") or die $!; my $line1 = <FH>; my $line2 = <FH>; print "eof!\n" if eof FH; my $line3 = <FH>; close FH; __END__ eof!
      That's what I meant: we need to be wary that a line read will not always return defined.

      That's exactly the condition when both the while($line = <FH>) { } and while(defined($line = <FH>)) { } loops terminate, so there's no difference in a while loop.

          Yes, but while takes care of that - and we/you were talking about a while loop, weren't you?

        Yes and no. I was talking about:
        while (<FH>) { ... }

        VS
        $line = <FH>; # on its own / not as while condition

        Given the OP's prelude, I really thought that the OP was referring to these two cases. Sorry for the confusion! :)

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